switch fully to gexec

Signed-off-by: Chris Adams <cadams@pivotallabs.com>
bump deps

Signed-off-by: Chris Adams <cadams@pivotallabs.com>
spawn iomux-spawn backgrounded

this prevents the server from hanging while being stopped in the runner.
previously stdout/stderr would be hooked into iomux-spawn which prevented
(*exec).Cmd from returning.

Signed-off-by: Chris Adams <cadams@pivotallabs.com>